Due to the prodigious quantities of thermal energy produced by fissioning nuclear fuel, it is imperative to maintain the fuel core of commercial water cooled nuclear fission reactors submerged within heat transferring coolant water. The conveyance of heat out away from the energy producing fuel core by means of a fluid coolant is needed to preclude the possibility of hazardous conditions or reactor damage such as could occur with an overheating meltdown within the fuel core unit.
Such a potentially destructive occurrence can result from a loss-of-coolant accident (LOCA) caused by an extensive breach of a major reactor coolant receptacle or conduit. To cope with this hypothetical accidental event, commercial water cooled nuclear fission reactors are provided with large reservoirs of water and dedicated safety injection systems(s) capable of supplying supplementary coolant water to the reactor vessel for cooling the fuel core and maintaining lower or normal operating temperatures. Typically, automatic safety measures are employed to activate and operate systems for supplying this supplementary coolant water as needed to the fuel core to replace or supplement the original coolant water lost due to some mishap.
A different but additional safety measure commonly employed in commercial water cooled nuclear fission reactors comprises means to deal with any failure of the reactor control rods to effectively perform their designed fission regulating function. Nuclear reactor control rods, comprising elongated units containing neutron absorbing material such as boron or a compound thereof, are designed to be reciprocally movable into and out from the fuel core of fissionable material. The level of fission activity of the fuel material in a core of a reactor, and in turn heat produced, is determined or controlled by the amount of neutron absorbent advanced into or withdrawn from the fuel core with the control rod units. Moreover, the fission reaction of the plant can be rendered subcritical or terminated by inserting sufficient neutron absorbent material housed within control rods into the fuel core to deprive the fuel of the needed quantity of fission produced neutrons for maintaining the self perpetuating fission reaction, or so-called chain reaction, and in turn heat produced.
However, in the event that the control rods fail to perform their intended role of governing and/or ceasing the fission reaction for any cause, mechanical, electrical or personnel malfunction, an auxiliary backup system is frequently provided for depriving the fuel core of the neutrons essential for maintenance of the heat producing fission reaction. Commonly this system comprises a supply of a water solution of a soluble boron or gadolinium compound, or so-called poison, which when introduced into the nuclear reactor vessel, merges with the reactor coolant and thus pervades the fuel core region to absorb the fission produced neutrons needed for continuing the fission reaction. Thus, in the remote event of any control rod failure, the poison solution is fed from a reservoir into the reactor vessel by suitable means on a signal of problem whereupon the fission reaction is terminated.
A typical arrangement in commercial water cooled nuclear fission reactor plants for incorporating the foregoing standby safety systems which inject auxiliary coolant water to temper the fuel core temperature, or inject a neutron absorbing solution to deprive the fuel of neutrons, utilized an apt gas, such as nitrogen, for a propellant to drive the liquid water or boron solution from its source or reservoir through communicating conduits into the reactor vessel. Thus, auxiliary coolant water or poison solution is maintained within a closed vessel or tank under sufficient gas pressure to drive the liquid contents into the reactor vessel through an appropriate arrangement of conduits upon a manually or automatically actuated signal responding to a malfunction.